Birthday Grudge: A compound word used to describe a person's anger as a result of others forgetting their birthday. As a result the victim retaliates by failing to acknowledge the birthdays of all of those who are guilty of the forsaking. This is considered to be petty behavior, and is often manifested on social network sites (e.g Facebook and Twitter).

OMG I cannot believe he is holding a birthday grudge against me. Afterall I did wish him a happy belated birthday.